In tool management, particularly in throw-away type cutting tools, it is desirable to have each edge of the tool monitored by a sensor that would detect tool wear or remaining tool life.
Various proposals have been made to measure tool wear by incorporating resistances for example onto the flank face of a cutting tool and monitoring the resistance of such resistors. A good summary of known system is provided in the paper entitled `A Study on Throw-Away Tool Equipped with a Sensor to Detect Flank Wear` by Aoyama et al. in the Bulletin of the Japan Society for Precision Engineering, Volume 21, No. 3, Sept. 1987, pages 203-208.
The various sensors described in this paper are thin film resistors that are applied to the flank face of a tool. The authors propose a different form of resistor that can be operated independent of the work piece, i.e. the work piece does not form part of the circuit to the resistor so that the resistance may be measured both on and off the machine.
All of the systems disclosed in the Aoyama et al. paper obtain a change in resistance of the incorporated resistor only by abrasion, i.e. the size of the conducting path is changed by removal of some of the resistor during the cutting operation of the tool thereby to provide a permanent record of the wear to which the tool has been subjected. In all of the systems with the exception of the one proposed by the authors of the paper, resistance can only be measured or the estimate of tool wear can only be made when the tool is in operation and the element to be cut is in position as the element forms part of the electric circuit necessary to measure the resistance.
Thick film resistors are well-known and it has been found that certain problems are encountered with the accuracy of the resistances changing over time when subjected to high temperature, thermal stress or high moisture conditions. This characteristic of certain thick film resistors was investigated and reported in a paper given in the proceedings European Hybrid Micro-Electronics Conference in 1979 entitled `Some Observations on the Accelerated Aging of Thick Film Resistors` by Sinnadurai et al. It was found that the resistance of certain thick films changed permanently when subjected to elevated temperatures over periods of time or to other conditions such as high humidity condition and that the degree of change in resistance or manner in which the resistance of the various thick films were changed was dependent on the ink used, resistor value (i.e. high or low resistance) and the manner in which the resistor was encapsulated.
It is also well known that the higher the tool temperature the faster the wear rate and thus the shorter the tool's life expectancy (see trans. A.S.M.E. Journal of Engineering for Industry Vol. 85 (1963) page 33.)